In various branches of the art, semiconductor devices, particularly semiconductor rectifiers of high current load-carrying capability, are more and more often used under extreme operating conditions due to their state of development. An extraordinarily heavy stress is placed on large-area semiconductor rectifiers, particularly when used with frequent changes between no load and nominal load. Changes in operating temperature with corresponding changes in load (due to the varying physical characteristics of the different materials employed) result in subjecting the layer construction of the semiconductor device to heavy tangential or shearing forces. These forces act primarily on the semiconductor wafer and on solder contact layers disposed between the semiconductor wafer and adjacent current-conductive components and often lead to malfunction of the semiconductor device after only a short period of operation. The effect of these stresses is dependent on the frequency of load changes and the thus occurring changes in operating temperatures as well as on the range of these differences in operating temperature.
In order to provide desired variable-load-resistant contacts between a semiconductor wafer and current-conductive components by means of a suitable semiconductor solder, it is advisable, particularly for technical production reasons, to provide the semiconductor wafer with metallic contact coatings of high mechanical stability before they are contacted.
There are thus semiconductor devices in which the semiconductor wafer is bonded to a contact plate by an alloying or soldering process, at least at one contacting surface. The contact plate preferably consists of a material, e.g. molybdenum and tungsten, which has a coefficient of thermal expansion approximately corresponding to the coefficient of thermal expansion of the semiconductor material.
Moreover, there are semiconductor devices having a semiconductor wafer provided with at least one metallic contact coating which is particularly suited to permit planar contact by means of soft solder.
In other semiconductor devices a contact coating (serving exclusively to improve wetting by a special semiconductor solder provided for the contact) is applied to the semiconductor wafer. This solder, which has high mechanical stability and a melting point below 450.degree.C, remains resilient even under the influence of varying mechanical stresses and will hereafter be referred to as a semiconductor hard solder.
Known semiconductor devices, however, do not seem to be suited for use with extreme load variations due to (a) insufficient compensation of mechanical stresses resulting from different coefficients of thermal expansion, (b) lack of mechanical stability and resiliency of the soft solders employed or (c) undesirable transfer of tangential or shearing forces from resilient semiconductor hard solder to the semiconductor wafer.
There are also semiconductor devices in which a semiconductor wafer, which is firmly connected to a molybdenum or tungsten wafer, is contacted by means of current-conductive components under pressure from spring members. In spite of good compensation for variations in thermal expansion provided by the construction of these known arrangements, this so-called pressure-contacting does not seem to be the best solution of the contacting problem in semiconductor devices, particularly in view of the requirement that thermal contact resistance be as low as possible.
German Published patent application Ser. No. 1,074,160 discloses a method for producing virtually blockage-free contact electrodes on semiconductor bodies in which, in order to achieve soft solder contacts for the application of leads, predetermined surface regions of the semiconductor body are provided with a first metal layer with an work function which approximates that of the semiconductor material. Onto this first metal a second metal is applied for the solder-attachment of leads with the requirement that, during the soldering process, an alloying take place only at the border area between the solder and the second metal, not at border areas between the second metal and the first metal or between the first metal and the semiconductor material. This problem particularly pertains to the provision of a blockage-free contact on small-area semiconductor devices, preferably to the base contacting of transistors.